The overflow of deep water from the Nordic Seas into the North
Atlantic plays a critical role in global ocean circulation and
climate. Approximately half of this overflow occurs via the
Iceland–Scotland (I–S) overflow, yet the history of its strength
throughout the Holocene (~0–11 700 yr ago, ka) is poorly
constrained, with previous studies presenting apparently
contradictory evidence regarding its long-term variability. Here, we
provide a comprehensive reconstruction of I–S overflow strength
throughout the Holocene using sediment grain size data from a depth
transect of 13 cores from the Iceland basin. Our results reveal
weaker I–S overflow during the early and late Holocene, with
maximum overflow strength occurring at ~7 ka, the time of
a regional climate thermal maximum. Climate model simulations
suggest a shoaling of deep convection in the Nordic Seas during the
early and late Holocene, consistent with our evidence for weaker
I–S overflow during these intervals. Whereas the reduction in I–S
overflow strength during the early Holocene likely resulted from
melting remnant glacial ice-sheets, the decline throughout the last
7000 yr was caused by an orbitally-induced increase in the amount
of Arctic sea-ice entering the Nordic Seas. Although the flux of
Arctic sea-ice to the Nordic Seas is expected to decrease throughout
the next century, model simulations predict that under high
emissions scenarios, competing effects, such as warmer sea surface
temperatures in the Nordic Seas, will result in reduced deep
convection, likely driving a weaker I–S overflow.